1. Field of the Invention
This invention is directed to electrical cable assemblies which include a cable with a stiff, preferably EMI/RFI shielding and/or environmentally secure, sleeve and a connector having contacts attached to wiring within the cable sleeve and which must be accessible for repair.
2. Background Information
There are many installations where electrical cables are subjected to very hostile operating conditions. One example is electrical cables used on gas turbine engines. Such cables must be capable of operating through a wide temperature range, for instance about -55.degree. C. to 200.degree. C. continuous operating temperature. They must also be resistant to aviation fuel, hydraulic oil, lubricating oil and deicing fluids. In addition, these cables must be moisture resistant, flexible, capable of withstanding vibration, and be shielded from electromagnetic interference (EMI) and radio frequency interference (RFI).
Electrical cable assemblies for combustion engines in particular should also be easily and economically repairable, not only at depot level, where more involved and tooling intensive repair can be performed, but also in the field. For effective repair in the field, the cable assemblies should be such that the tools required to facilitate the repair can be limited to those tools commonly supplied with replacement connectors or found in a typical tool box. The need for electrical power to facilitate repair should be avoided. In addition, the degree of difficulty in performing the repair should be minimized. The procedure should also be concise and straightforward, eliminating the need for detailed instructions. Finally, the number of individual components that must be removed or replaced in the repair procedure should be minimized, and loose parts that could become separated from the assembly should not be used.
Presently, there are four basic design concepts for repairable cable assemblies used in hostile environments. None of these four designs meet all of the above requirements for repairable cable assemblies. The four basic design concepts are: (1) open bundles, (2) conduits/raceways, (3) braid socks, and (4) shrink sleeve boots.
Open bundle designs are composed of shielded complexes, twisted shielded pairs, triples, quads, etc., laid up into a cable assembly. These complexes are then bundled by lacing ties and installed on the engine. The connectors are easily accessible for removal and replacement of damaged contacts and the entire circuits can be removed and replaced easily. While these open bundle cable assemblies offer a high degree of repairability, are light in weight, flexible and the lowest in cost, their EMI/RFI immunity is not always sufficient, and moisture resistance and abrasion resistance are poor.
Conduit, or raceway designs are another way of building cable assemblies with a high degree of repairability. These designs have an increased resistance to abrasion, moisture and EMI/RFI interference as opposed to open bundled designs. They are similar to open bundle designs with the exception that the individual complexes are routed through conduit to the connectors. By conduit, it is meant continuous tubing. The conduit can be either metallic or non-metallic/shielded. Specially designed backshells and breakout transitions are used that allow the conduit to be disconnected at the connector or breakout transition facilitating repair.
Excess wire, one to two inches is designed into each of the circuits of the conduit/raceway assembly. This extra wire is stored in the conduit backshell at the time of assembly. This allows the connector to be extended out away from the conduit/backshell assembly during the repair procedure permitting access to the contacts for removal.
The advantages to the conduit/raceway system are: excellent repairability, only tools normally supplied with the connectors are required for repair, entire circuits can be removed or replaced, abrasion resistance is good, they can be made moisture proof, and EMI/RFI immunity is good. On the other hand, the backshells, adaptors and breakout transitions used to connect the conduit to the connectors and the conduits themselves add significant weight to the cable assembly. The conduit restricts flexibility, the cables within the conduit can become excited in response to vibration with a high probability of abrasion of conductor insulation, and this is the most costly of the cable assemblies.
Braid socks are another method of designing in repairability to a shielded cable assembly. The outer braided wire shield is purposely braided loosely over the last two to three inches of the cable that attaches to the connector backshell. This loosely braided portion allows the backshell to be removed from the connector and forced back upon the cable to provide accessibility for contact removal or repair.
Braid socks that are actually two layers of loosely braided material can be purchased already attached to connector backshells. These sections of braid are then soldered to the end of the braid on the cable assembly before being attached to the connector. The double layer of loosely braided shield material is used to reduce the number of "holes" that appear in the braid as a result of the loose braiding. These "holes" allow electromagnetic and radio frequency signals to enter and leave the cable unattenuated.
Braided sock cable assemblies have good repairability, require only tools normally supplied with the connector, are relatively light weight, are abrasion resistant when stainless steel is used as the outer braid and flexibility is good. Although not as flexible as open bundle type cables, the reduction in flexibility is a result of the added EMI/RFI shielding, and not the braid sock. Disadvantages of the braided sock assembly are: they cannot be easily moisture proofed, EMI/RFI shielding is degraded with each repair, and entire circuits cannot be replaced. In terms of value for the dollar, this type of assembly is very cost effective.
The shrink sleeve boot design, which has been recently introduced, combines the braid sock principle with the use of heat shrinkable connector boots and cable jacketing. The heat shrink sleeving is applied over the cable outer braid. A heat shrinkable boot joins the connector backshell to the cable and seals onto the cable heat shrink jacket. Beneath this boot is the loosely braided sock. Repair is accomplished by reheating the heat shrinkable boot and a section of the cable jacketing. The connector backshell is then removed from the connector. The reheated boot is flexible enough so that the backshell braid sock and heat shrink cable jacketing is capable of being pushed back enough to allow access to the connector for repair. The shrink sleeve boot design has the same advantages as the braided sock design with the addition that the shrink sleeving makes the cable moisture proof. This type of design has a disadvantage that a heat gun is required for repair and this requires electric power. In addition, EMI/RFI immunity is subject to the same degradation as a result of repair as discussed in connection with the braid sock design. Furthermore, the braided shield can cause damage to the shrink sleeve jacket during repair thereby degrading moisture resistance. The application of heat causes the shrink sleeving materials to loose most of their tear resistance. This design for a repairable cable assembly is also quite costly.